This invention relates to a variable valve timing controller to control the valve timing of an internal combustion engine.
A conventional variable valve timing controller comprises: a rotational shaft for opening and closing a valve; a rotation transmitting member rotatably mounted on the rotational shaft; a vane supported by the rotational shaft; a pressure chamber formed between the rotational shaft and the rotation transmitting member and divided into an advance chamber and a delay chamber by the vane; an advance fluid passage communicated with the advance chamber for supplying and discharging an operational fluid; a delay fluid passage communicated with the delay chamber for supplying and discharging the operational fluid; and a locking mechanism for maintaining a relative position between the rotational shaft and the rotation transmitting member. Such a conventional variable timing device is disclosed, for example, in Japanese Patent Laid-Open Publication No. 01-92504 published in Japan on Apr. 11, 1989 (corresponding to U.S. Pat. No. 4,858,572 issued in the United States on Aug. 22, 1989, the entire disclosure of which is incorporated herein by reference) and in Japanese Patent Laid-Open Publication No. 09-250310 published in Japan on Sep. 22, 1997.
In the conventional variable valve timing controller, the valve timing is advanced due to relative rotation between the rotational shaft and the rotation transmitting member when the operational fluid is supplied to the advance chamber and is discharged from the delay chamber. On the contrary, the valve timing is delayed due to the opposite rotation between the rotational shaft and the rotation transmitting member when the operational fluid is discharged from the advance chamber and is supplied to the delay chamber.
Further, in the conventional variable valve timing controller disclosed in the above-mentioned publications, the vane transmits torque from the rotation-transmitting member to the rotational shaft. Therefore, the rotational shaft always receives a counter torque to expand the delay chamber while the internal combustion engine runs. When the internal combustion engine stalls, due to the counter torque, the rotational shaft rotates to expand the delay chamber since pressure of the operational fluid is insufficient to hold the vane at the current position. Thus, the rotational shaft reaches the most delayed position where the delay chamber is the most expanded. In case the internal combustion engine is restarted at the most delayed position of the rotational shaft, due to unstable transitional pressure, the vane vibrates and generates undesirable noise. Conventionally, the locking mechanism maintains the predetermined relative position between the rotational shaft and the rotation-transmitting member so that generation of vibration of the vane is somewhat prevented.
By the way, air intake tries to flow into a cylinder of the internal combustion engine by inertia even after the piston begins to go to the top dead center while the internal combustion engine runs at high speed. Therefore, volumetric efficiency may be improved by delayed closure of an air-intake valve so that the output of the internal combustion engine may be improved.
However, in the conventional variable valve timing controller, the most delayed timing has to be set so that the air intake is sufficient to start the internal combustion engine. This means that the closing timing of the air-intake valve is not optimized for the high-speed operation of the internal combustion engine. Thus, the volumetric efficiency cannot be improved by the inertia of the air intake. If the closing timing of the air intake valve is unreasonably optimized for the high-speed operation of the internal combustion engine, the air intake which is once inhaled into the cylinder flows backward upon start of the internal combustion engine since the air intake does not have enough inertia and the air-intake valve continues to be opened even after the piston passes the bottom dead center and begins to go to the top dead center. Therefore, the internal combustion engine becomes hard to start due to insufficient compression ratio and imperfect combustion. Further, in the conventional variable valve timing controller, due to low atmospheric pressure, a similar disadvantage may be expected at altitudes if the air intake valve is set to be closed at around the bottom dead center of the piston.
Further, in the conventional variable timing controller, if the exhaust valve timing is delayed similarly, an amount of exhaust gas recirculation is increased by an extended overlapping time of the air-intake valve and the exhaust valve so that the internal combustion engine becomes hard to start.